The studies address one of the most intractable problems of higher education: the dead end of remedial education. At most community colleges, a majority of entering students who recently graduated from high school are placed in remedial classes, where they pay tuition but earn no college credit. Over all, less than a quarter of those who start in remedial classes go on to earn two-year degrees or transfer to four-year colleges.

“We hear a lot about the high rates of failure in college-level classes at community colleges,” said Judith Scott-Clayton, the author of the urban study and a Teachers College professor of economics and education and senior research associate. “Those are very visible. What’s harder to see are the students who could have done well at college level but never got the chance because of these placement tests.”

The colleges’ use of the leading placement tests — the College Board’s Accuplacer and ACT’s Compass — lead to mistakes in both directions, the studies find, but students going into college-level classes they cannot handle is not as serious as unnecessary remedial placement, which often derails college careers.

Although the placement tests have been widely used since the late 1980s, students rarely understand how much is at stake. Typically, students are told that they need not worry about the tests because they are for placement — and very few colleges encourage them to prepare as they would for a college-entrance exam like the SAT.

The studies found that using high school grade-point averages as the basis for placement would be as good as or better than using the placement tests, but the authors stopped short of recommending that community colleges simply drop the tests and use high school transcripts when available.

“It’s probably a mistake to rely on any single measure for high-stakes decisions,” said Clive Belfield, who is an economics professor at Queens College, a researcher at Teachers College and one of the authors of the study on the statewide system. “Where you have both a test and a high school transcript, the best thing is to use both together.”

Remedial education practices vary widely. At some colleges, even if remedial courses are recommended, students can choose to register for college-level courses; at others, the courses are mandatory for those below the cut-off scores. With the Obama administration pushing to improve the nation’s dismal community college graduation rates, many states and community college systems are rethinking their approaches to remedial education.

“I haven’t seen the studies, but what I do know is that when I talk with leaders of community colleges, a lot of them have issues with the diagnostic tests and sense that far too many students are being put in developmental, remedial education, especially in math,” said Walter G. Bumphus, president of the American Association of Community Colleges. “Almost every one of them has some plan to change that.”

In Virginia, for example, Northern Virginia Community College recently modularized its math requirements so that students can study just the areas in which they are weak, and not be stuck in semester-long math classes.

In addition, the math requirements differ depending on a student’s academic program. The English faculty, too, is re-examining its remedial program.

At Lake Area Technical Institute in South Dakota, each of the 27 majors has different admissions standards, so that, for example, precision-machining students need higher math scores than those studying cosmetology.

“We get some students with rusty math skills who do poorly on the test, and we send them to a Web site where they can brush up their skills and take the test again, and most of them do fine,” said Deb Shephard, Lake Area’s president. “It’s less than 5 percent of our entering students who need remediation, and they do it on their lunch hour, side by side with the other courses they’re taking.”

A version of this article appears in print on February 29, 2012, on page A19 of the New York edition with the headline: Colleges Misassign Many to Remedial Classes, Studies Find. Order Reprints|Today's Paper|Subscribe